1. Technical Field
The present disclosure refers to Class-G power amplifiers, and more particularly, to an output stage for Class-G power amplifiers. The present description further refers to an audio amplifier system.
2. Discussion of the Related Art
Class-G power amplifiers are known, made of one or more output stages supplied with different voltages, the activation of which dynamically occurs, in conformity with the amplitude of the input signal.
In Class-G power amplifiers, the signal is amplified from the stage supplied to a voltage immediately greater than the instantaneous amplitude of the input signal itself, so as to minimize the voltage drop in the output stage, when it draws the load current.
A reduced power dissipation and a greater power efficiency are obtained in comparison with that of an amplifier working as a Class-B and/or Class-AB amplifier, in particular with signals with high crest factor, like audio signals, which extend around zero for most of the time, with few excursions at high level.
Some useful considerations regarding the performance of Class-G amplifiers are shown in following documents:
F. H. Raab—Average Efficiency of Class-G Power Amplifiers—IEEE Transactions on Consumer Electronics, Vol. CE-32, No. 2, May 1986;
R. van der Zee, “High Efficiency Audio Power Amplifiers: Design and Practical Use” 1999 available Online at the address: www.ub.utwente.nl/webdocs/el/1/t000000d.pdf
The power stages of the Class-G amplifier are configured according to two basic topologies: the stacked (serial) topology and the shunt (parallel) topology.
Some examples of power stages for Class-G amplifiers, with stacked topology, are described in documents U.S. Pat. No. 3,961,280 and U.S. Pat. No. 3,772,606.
Some examples of power stages for Class-G amplifiers with shunt topology, are described in following documents:                D. H. Horrocks—Active filter power dissipation reduction using improved output stage—Digital and Analog Filters and Filtering Systems, IEE Colloquium on, 25 May 1990;        D. Self—Audio Power Amplifier Design Handbook—News, Chapter 10.        
With reference to D. H. Horroks documents cited before, they describe one shunt-topology Class-G amplifier (FIG. 1(a)) and one with stacked topology (FIG. 1(b)), each having two BJT Class-B stages. In such document two symmetrical supplies are made: one first voltage having a lower value, ±VL, and one second voltage having a greater value, ±VH. The first Class-B stage has two transistors (Q1 and Q2) with a push-pull configurations, which are supplied by the lower voltages ±VL and the second Class-B stage is provided with other two transistors with a push-pull configuration (Q3 and Q4) supplied by the greater voltages ±VH.
During the operation of the amplifiers described in the article of D. H. Horroks when the amplitude of the input signal is lower than a ±VL value, the current to be supplied to a load is generated by those transistors (Q1 and Q2) which are supplied by the lower voltage ±VL. When the amplitude of the input signal further increases, the electric current at the load is supplied after a switching phase, by the transistors Q3 and Q4 supplied by the greater voltages ±VH. In particular, for the shunt topology the transistors Q1 and Q2 are turned off when the transistors Q3 and Q4 are in operation, whereas for the stacked topology, the transistors Q1 and Q2 remain powered and are isolated from lower supplies ±VL by means of diodes.
Class-G amplifiers of the known type have a problem which has limited their use in audio “high fidelity” applications: such problem being the distortion of the waveform at the switching among the various amplifier stages, also known as “switching noise”.
Such switching noise is particularly present at high frequencies from the audio-band, as noted in document of T. Sampei at al.—Highest efficiency and super quality audio amplifier using MOS power fets in class G operation—IEEE Transactions on Consumer Electronics, Vol. CE-24, No. 3, August 1978. Such document proposes the use of MOSFET-type power transistors.